1. Field of the Invention
The invention relates to a protective device for a water-pump apparatus, more particularly to a protective device which can prevent idle running of a motor unit of a water-pump apparatus.
2. Description of the Related Art
A water-pump apparatus is usually installed in high-rise buildings so as to pump water from a ground storage reservoir to an elevated tank in order to permit the distribution of water to households of the high-rise building.
Presently, a pump unit of the water-pump apparatus may be operating even when there is no water input thereto, thereby resulting in idle running of a motor unit of the water-pump apparatus. The following are some of the reasons why idle running of the motor unit occurs:
1. The water level of the ground storage reservoir should be above a minimum level in order to ensure proper operation of the water-pump apparatus. Idle running of the motor unit can occur when the water level is below the minimum level and when a water level switch, which is used to control the supply of the power to the motor unit, in the ground storage reservoir is damaged.
2. When water supply from the local water service company is cut-off, and there is no ground storage reservoir, the water-pump apparatus is unable to draw water, thus resulting in idle running of the motor unit.
3. A pipeline interconnects the ground storage reservoir and the water-pump apparatus. When the pipeline is damaged, water does not reach the water-pump apparatus, thus resulting in idle running of the motor unit.
Note that rotation of the rotary parts of the pump unit is continued when the motor unit is running idly. Water serves as a lubricant for the rotary parts. Therefore, when no water is present in the pump unit, locking of the rotary parts can occur. Overloading of the motor unit is possible, thus resulting in overheating and damage to the stator of the motor unit.
In order to overcome the above drawback, a protective device is mounted on the water-pump apparatus to prevent idle running of the motor unit. Referring to FIG. 1, a conventional protective device for a water-pump apparatus includes a pair of bar electrodes (23) which extends into a three-way pipe (22). The bar electrodes (23) are used to detect if water is entering the pump unit (12) of the water-pump apparatus (1). The bar electrodes (23) are connected to a control unit (21) which controls the operation of the motor unit (11) to prevent idle running of the latter. The three-way pipe (22) is connected to the inlet pipe (20) of the pump unit (12). When the three-way pipe (22) has water therein, the pump unit (12) similarly has water therein. There is no resistance across the bar electrodes (23) at this stage, and thus, the control unit (21) permits electrical connection between a power supply (25) and the motor unit (11). Accordingly, when the three-way pipe (22) has no water therein, there is a large resistance across the bar electrodes (23), and thus, the control unit (21) disconnects the power supply (25) and the motor unit (11), thereby preventing the operation of the latter.
It has thus been shown that in the conventional protective device, the presence of water is detected in order to determine whether the control unit (21) should make or break electrical connection between the power supply (25) and the motor unit (11). In order to detect the presence of water, a pair of bar electrodes (23) extends from the bottom of the control unit (21). Therefore, the bar electrodes (23) are normally immersed in water for long periods of time. After a period of use, foreign objects (such as moss) collect at the space between the bar electrodes (23) and can affect the detecting ability of the bar electrodes (23). For example, assuming that the foreign article which gathered between the bar electrodes (23) possesses good electrical conductivity, when the three-way pipe (22) has no water therein to indicate that there is no water in the pump unit (12), the resistance across the bar electrodes (23) is very small, and thus, the motor unit (11) still continues to operate.
Another drawback of the conventional protective device is that it cannot be installed conveniently. During installation, the inlet pipe (20) of the pump unit (12) should be cut to an appropriate length before the three-way pipe (22) can be connected thereto. The control unit (21) is then assembled on top of the three-way pipe (22).
FIG. 2 is a schematic electrical circuit diagram of the control unit (21). The control unit (21) is shown to comprise a transformer, diode rectifiers, voltage regulators and some integrated circuits. The manufacturing cost of the control unit (21) is relatively high because of the relatively large number of circuit components involved. Furthermore, repair of the control unit (21) cannot be accomplished easily because of the large number of circuit components.
It should also be noted that since the bar electrodes (23) are in direct contact with water, water may seep into the control unit (21) if the latter was not sealed properly, thereby causing short-circuiting of the control unit (21).